I always choose to be has evil has I can without wasting the game, like on fallout 3 I do the quests, then I kill the quest giver and steal all their stuff. I always insult, lie and steal, it just feels more fun being an evil bastard to me. But afterwards I play it again has a goody.
I remember in fallout 3 I decided to blow up megaton, but before I did it I went to megaton, killed everyone and robbed all their houses and body's the only thing I regret was that I didn't realise there was a bobble head in the sheriff's house, my friend thought I was a dick for doing that, but I don't care I do it to every town I find unless I like the characters. The only person I didn't kill in megaton though was the person I hated most, the one which made me swim in toxic waste, I killed her and reloaded over and over again but when it came to it I didn't want to kill her but I robbed her and pick pocketed all her stuff instead.